What genocide? Burning/destroying an enemy city in war, is an act of war.
And yes we were already at war, because it started with an Alliance rapid dog named Greymane attacking the horde’s fleet in stormheim. The motives are irrelevant in this case. An act of war is an act of war, and warrants a response.
The only reason the response wasn’t immediate was because the entire world was dealing with the legion invasion threat at the time when Anduin couldn’t keep his rapid dog general on a leash. The large scale attack on the horde fleet in stormheim ended whatever agreements/treaties the 2 factions had before, end of story.
Sorry to burst your bubble, but no, the burning of the tree/city of the nightelves was a simple act of war. Innocents die in wars, deal with it, or switch to another game where War isn’t part of it’s name. Try animal crossing or anything from Nintendo, I’m sure you’ll find them more suiting.
Good day. 